1. Field of the Invention
This invention relates to a flywheel-type power storage device which stores electrical energy by converting the electrical energy to flywheel rotation energy. This device is useful for backing up the important power sources provided for emergencies and is also useful in combination with power generation by sunlight, wind force, wave force or the like to level the output electric power thereof.
2. Prior Art
There has heretofore been known an electrical energy storage device provided with a rotary electrical machine (or motor/generator) and a flywheel fixed to the rotor of the machine. The rotary electrical machine which has been used in the conventional flywheel-type power storage device, however, is not suitable for use for power storage due to its energy loss.
For example, Takahashi et al. (U.S. Pat. No. 5,065,060) disclose a flywheel type energy storage device comprising an induction motor and a flywheel. It is well known to one skilled in the art that the power loss of an induction motor is larger than a dc or synchronous motor having a similar dimension thereto. More specifically, in Takahashi's device, inductive loss (or iron loss) incurs in the rotor upon accumulation (or charge) and release (or discharge) of electric power although it does not incur because of the absence of excitation upon racing of the motor after accumulating the electric power. Since a high efficiency flywheel is placed in a vacuum or in vacuo to prevent the flywheel from causing its energy loss due to friction between it and the air, the motor assembled in one body with the flywheel is also placed in the vacuum and it is very difficult to sink or remove heat generated in the rotor. Accordingly, it is difficult to make such a device with a large capacity and, therefore, such a device is limitedly used for a relatively small capacity one such as UPS.
Hall et al. (U.S. Pat. No. 4,713,554) disclose a low profile ring-shaped motor comprising a stator having a winding with a core and a rotor having inner and outer field members constituting therebetween a field gap in which the winding wound on the core is positioned. One of the motor disclosed by Hall et al. is a dc/synchronous one whose field members comprise permanent magnets. It is considered to use a dc/synchronous motor disclosed by Hall et al. instead of the induction motor of the flywheel type energy storage apparatus disclosed by Takahashi et al. in order to reduce the motor loss. Then, there is raised a problem as to bearings being great in power loss.
In such a device which uses a motor, a ball-and-roller bearing and a magnetic bearing are used as a bearing to bear the rotation axis of the rotor. Such a device in which a magnetic bearing is used, is disclosed in, for example, "27th Intersociety Energy Conversion Engineering Conference '92, Design and Manufacturing for a Composite Multi-Ring Flywheel" and French Patent No. 4,731,554 (Pierre).
The above bearings and the like will not be suitable for high-speed rotation as far as the conventional synchronous motor provided with winding having a core and a field member comprising a permanent magnet is used. More particularly, the deviation of rotation axis of the rotor causes unbalance of magnetic attraction exerted on the core of the winding of the rotor whereby the deviation of the rotor is further expanded, this being a vicious cycle. It thus becomes necessary to rigidly support the rotor very securely in order to prevent the rotor from being deviated thereby to suppress said vicious cycle, but this necessity is very technically difficult to realize, will cause the device to be complicated in structure and will incur a great power loss. Accordingly, the motor having the core therein comes to have a low limit on the rotation velocity allowable for such a motor whereby the motor is unsuitable for efficient storage of electrical power.
In addition, a dc/synchronous motor with core is unsuitable for a power storage device because it has a demerit that it generates iron loss even when it races after accumulation of power and continuously spends the accumulated or stored power, that is, it is low in power storing efficiency.
The magnetic bearing disclosed by Pierre (French Patent No. 4,731,554) achieves an electric servo control with intent to suppress said vicious cycle and, therefore, there will incur a great power loss caused by an electrical power for the servo control. Pierre further discloses a structure of coreless and brushless-type dc/synchronous motor using an inverter to generate a rotating magnetic field and employing a method wherein non-magnetic members are arranged between multipolar magnets to avoid generation of torque in the reverse direction to rotation direction, which will be generated in a conventional brushless-type motor. In Pierre's motor, however, since electro-magnetic power is not generated in wires near the non-magnetic members, the normal torque is reduced as far as the reverse torque is reduced.